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its short roots, cannot flourish in temporarily waterless conditions, whereas trees and even forest undergrowth, with much deeper roots, can tap into damper ground hidden well below the surface. Additionally, any clouds that were around would have tended to shed their rain when they arrived at the mountains. Even in drought conditions, mountainous areas normally receive more precipitation than adjacent plains.
    The Turkic economy was also much more varied than that of the Avars. The Turks had, for instance, an involvement with hunting and gathering, with mining and metalworking, and almost certainly with goats, sheep, horses, and above all cattle. 5 The Avar economy, on the other hand, revolved predominantly around raiding, sheep, and horses. To the Avars, the horse was everything—a source of meat, milk, cheese, yogurt, and even alcohol (the sweet fermented mares’ milk called
koumis
). Moreover, the horse was the vital ingredient in Avar military power. It was what had made their ethnic group top dog in the Mongolian region for 150 years.
    And yet the horse was also the Avars’ Achilles’ heel. Due to important differences in their digestive systems, horses often find it much more difficult to survive drought than cattle do. Horses fail to digest—and therefore they excrete—up to 75 percent of the protein they eat. By contrast, cattle excrete as little as 25 percent of the protein they consume. Thus, when all there was to eat was dried-out, low-protein grass, cattle had a marked advantage over horses.
    The high-protein wastage rate that horses suffer from is a result of the design of their digestive system. Both cows and horses have what are essentially fermentation vats inside them to convert plant protein into a usable source of energy. But the horse’s fermentation vat is less useful because it is located at a point where the food that passes through has already been digested. By contrast, the cow’s fermentation vat is located at a point where the food has not yet been digested.
    In the cow, most plant protein is broken down by bacterial action inside the organ known as the rumen. It is the only way in which the bulk of the plant protein can be utilized, because much of it is locked up in the cell walls of the plant. The freshly broken-down protein then passes into the cow’s duodenum, where it is broken down further into amino acids. These in turn pass into the small intestine, where they are absorbed into the bloodstream and used to make muscle, produce milk, repair damaged tissue, and nourish fetuses. The horse, however, has its fermentation vat located in its hind gut—well after the food has already passed through the duodenum and the small intestine. The horse therefore does not produce large quantities of amino acids and does not absorb large quantities of protein through its intestine walls. Instead, the plant protein it eats is broken down by bacterial action in an almost totally useless location, and all the animal succeeds in doing is depositing extremely nitrogen-rich excrement. This is good for the soil, but not of much short-term benefit to the horse.
    In normal climatic conditions, plant protein is so plentiful that neither horses nor cows need to retain all the protein they eat. But in drought conditions, when plant protein is rarer, while living grass contains around 15 percent protein, dead grass has only 4 percent; this makes a high protein absorption rate key to survival. The cow’s much more efficient protein extraction system succeeds in retaining three times the amount of protein as the comparatively useless horse system.
    Although horses are obviously of greater military use than cattle, their poorer survival ability in times of prolonged drought or severe winters put their Avar owners at a terrible disadvantage vis-à-vis the Turks. Even one hard winter coupled with an ultradry summer could kill large numbers of Avar horses. 6 Two or three successive bad years could create terrible